docs: def add_key_filters(self, key_filters):
          """
          Adds key filters to the inputs.
  
          :param key_filters: a list of filters
          :type key_filters: list
          :rtype: :class:`RiakMapReduce`
          """
          if self._input_mode == 'query':
              raise ValueError('Key filters are not supported in a query.')
  
          self._key_filters.extend(key_filters)
          return self